Names changed to protect the guilty

[Ben Finney]

"Andy Salnikov" <a_salnikov at yahoo.com> writes:

> "Aahz" <aahz at pythoncraft.com> wrote:
> > Antoon Pardon <apardon at forel.vub.ac.be> wrote:
> >>The problem is there is also ground for bugs if you don't use
> >>"blah is True". If some application naturally seems to ask for a
> >>variable that can be valued False, True or a positive integer then
> >>things like "if var" or "if not var" may very well be a bug too.
> >
> > Anyone designing an app like that in Python deserves to lose.
> > It's just another way of shooting yourself in the foot.
>
> OK, I guess nobody ever heard about three-valued logic before,
> right?

Three-valued logic is fine for some purposes. Bending boolean
two-valued constants to play the part of three-valued is confusing and
wrong.

> Of course it does not apply to the original post because has_key()
> can only return True or False (I hope it will not ever return
> DontKnow:) but in general if you implement something like 3-valued
> logic choices like (False,True,None) are almost obvious.

No. Using False and True is a strong signal to the reader that you're
using *two* value logic. If you break that expectation, the reader
should not be expected to sympathise.

As another poster suggested, if you want to implement three-valued
logic, use three new objects to represent the states, so the reader
*knows* there's something going on other than two-value logic. Don't
re-use False and True in three-valued logic and expect anyone to
understand your code.

-- 
 \       "It is seldom that liberty of any kind is lost all at once."  |
  `\                                                     -- David Hume |
_o__)                                                                  |
Ben Finney